Image coding control method and device

ABSTRACT

Image coding having high coding efficiency is provided with a small computational complexity by properly determining a prediction mode and a quantization parameter. When an image is coded by a prediction mode selected from plural prediction modes every any coding unit, an undetermined multiplier λ is first calculated from a quantization parameter (function S 301 ) Subsequently, the cost of each of R-D points (pairs of number of coded bits and coding distortion that correspond to plural combinations of prediction modes and quantization parameters) is calculated (function S 305 ) on the basis of the undetermined multiplier λ while generation and estimation of the R-D points and deletion of the points (functions S 302 -S 304 ) are repeated every coding unit, for example, every macroblock, and the optimal combination of the prediction mode and the quantization parameter is determined on the basis of the R-D point providing the minimum cost.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2004-372357, filed on Dec. 23,2004; the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates image coding control method and device formotion pictures or still images.

BACKGROUND OF THE INVENTION

According to a motion picture coding standard system such as ITU-TH.261, H.263, H.264, ISO/IEC MPEG-2, MPEG-4 part 2 or the like, codingis carried out every coding unit called as a macroblock while switchinga prediction mode. It is said that it greatly affects the codingperformance how the switching operation is carried out.

One of the methods of switching the prediction mode is disclosed in TMN9which is a test model of ITU-T H. 263 (see Non-patent Document 1, ITU-T,Study Group 16: “Video Codec Test Model, Near-term, Version 9 (TMN9)”,Document: Q15-C-15, 1997) In the Non-patent Document 1, threshold valueprocessing is carried out by using the absolute value difference sumbetween an input picture and a predictive picture achieved by motioncompensation, and the absolute value difference sum between the inputpicture and the macroblock of the input picture, whereby threeprediction modes of an inter-frame prediction mode of 8×8 blocks, aninter-frame prediction mode of 16×16 blocks and an intra-frameprediction mode are switched to one another.

Furthermore, a method of selecting the prediction mode of eachmacroblock on the basis of Lagrange's undetermined multiplier method isintroduced in a non-patent document 2 (Gary J. Sullivan and ThomasWiegand, “Rate-Distortion Optimization for Video Compression”, IEEESignal Processing Magazine, Vol. 15, No. 6, pp 74-90, November 1998).More specifically, coding is actually carried out in each predictionmode to determine a number of coded bits (rate) and a coding distortion.Thereafter, the cost of each prediction mode to the same quantizationparameter is calculated under the condition that the Lagrange'sundetermined multiplier is assumed as the function of the quantizationparameter, and the prediction mode in which the cost is minimum isselected. This publication reports that the coding efficiency is moregreatly enhanced as compared with TMN9 by this method.

The switching method of the prediction mode disclosed in the non-patentdocument 1 is an easy method, and the switching operation of theprediction mode can be performed with small computational complexity.However, this method pays no attention to the actual number of codedbits (rate) and the distortion, and thus it can be hardly said that theswitching operation of the prediction mode which is optimal in therelationship between the number of coded bits and the distortion iscarried out. Accordingly, this method does not greatly enhance thecoding efficiency.

According to the method disclosed in the non-patent document 2, a numberof coded bits—a coding distortion function having the same gradient withrespect to the same quantization parameter is assumed. Therefore, whenthe quantization parameter is varied or when the shape of the number ofcoded bits—coding distortion function is different from the assumedshape, it is impossible to carry out accurate comparison estimation onthe coding cost, and thus a proper prediction mode cannot be selected.Furthermore, it is also impossible to compare the coding cost when thequantization parameter is varied under the combination with the numberof coded bits control. Accordingly, it cannot be expected that thecoding efficient is greatly enhanced by this method.

Therefore, Japanese Patent Application No. 2004-177190 discloses amethod of selecting not only an optimal prediction mode, but also anoptimal quantization parameter. According to this method, not only theprediction mode of each macroblock, but also the value of thequantization parameter is varied to calculate the cost, and theprediction mode and the quantization parameter that provides the minimumcost are selected. This method more greatly enhances the codingefficiency as compared with the non-patent document 2. However, thefrequency at which the coding is actually carried out by using eachprediction mode and each quantization parameter is large, and thus thecomputational complexity may be increased.

Therefore, an object of the invention is to provide image coding controlmethod and device in which image coding having high coding efficiencycan be carried out with a small computational complexity by properlydetermining a prediction mode and a quantization parameter.

BRIEF SUMMARY OF THE INVENTION

According to embodiments of the invention, an image coding controlmethod for controlling a quantization parameter by using one predictionmode selected from plural prediction mode every any coding unit,comprises: calculating an undetermined multiplier corresponding to aparameter used for a cost function from a given reference quantizationparameter; selecting a set of secondary number of coded bits—codingdistortion pairs from a set of primary number of coded bits—codingdistortion pairs that correspond to plural combinations of theprediction mode and the quantization parameter every coding unit;selecting number of coded bits—coding distortion pairs providing theminimum cost function minimum as the optimal number of coded bits—codingdistortion pairs from the set of the secondary number of codedbits—coding distortion pairs every coding unit by using the undeterminedmultiplier thus calculated; and selecting a prediction mode and aquantization parameter used for the coding from the optimal number ofcoded bits—coding distortion pairs, wherein when the set of thesecondary number of coded bits—coding distortion pairs is selected everycoding unit,

(1) a convex hull comprising the set of the primary number of codedbits—coding distortion pairs is formed on a two-dimensional orthogonalcoordinate system in which the number of coded bits is set as one axisand the coding distortion is set as the other axis,

(2) a set on the convex hull that connect the position of a number ofcoded bits—coding distortion pair providing the minimum number of codedbits and the position of a number of coded bits—coding distortion pairproviding the minimum coding distortion is determined, and

(3) a set of number of coded bits—coding distortion pairs that is on theconvex hull and exists at a nearer side to the origin of thetwo-dimensional orthogonal coordinate system is selected as a set of thesecondary number of coded bits—coding distortion pairs.

According to the embodiments of the invention, by forming the set of thesecondary number of coded bits—coding distortion pairs for sequentiallyforming the convex hull, the prediction mode and the quantizationparameter to be used for the coding can be determined from the optimalnumber of coded bits—coding distortion pairs while considering theoverall computational complexity. Accordingly, the proper predictionmode and the quantization parameter are determined with a smallcomputational complexity, whereby an image coding operation can beperformed with high coding efficiency.

Furthermore, when a set of secondary number of coded bits—codingdistortion pairs is selected, the set of the primary number of codedbits—coding distortion pairs located in the convex hull has a lowprobability to minimize the cost, and thus they are sequentially deletedfrom candidates, whereby the computational complexity is expected to bereduced without lowering the coding efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the construction of an image codingdevice according to an embodiment of the invention;

FIG. 2 is a block diagram showing the construction of a framememory/predictive image creator in FIG. 1;

FIG. 3 is a flowchart showing the processing flow of a coding controllerand a mode selector of FIG. 1;

FIG. 4 is a diagram showing an R-D point that is encoded by a referencequantization parameter;

FIG. 5 is a diagram showing a specific example of a method of findingout a lower left convex hull to determine an R-D point set;

FIGS. 6A and 6B are diagrams showing a specific example of a method offinding out a lower left convex hull to determine the R-D point set;

FIG. 7 is a diagram showing the processing of deleting a prediction modethat does not exist at the lower left convex hull in the R-D point set;

FIG. 8 is a diagram showing a process of estimating a number of codedbits and a coding distortion from actually-measured values;

FIG. 9 is a diagram showing a process of selecting an R-D point havingthe minimum cost from the R-D point set; and

FIG. 10 is a diagram showing an R-D plane to explain a left convex hull.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments according to the invention will be described hereunder withreference to the accompanying drawings.

(1) Construction of Image Coding Device

FIG. 1 shows an image coding device for coding a motion pictureaccording to an embodiment of the invention.

A motion picture signal is input as an image signal 100 to the imagecoding device on a frame basis.

The difference between an input image signal 100 and a predictive imagesignal 102 is taken by a subtracter 101 to generate a prediction errorsignal 103. On the prediction error signal 103 thus generated,orthogonal transformation, for example, discrete cosine transformation(DCT) is conducted by an orthogonal transformer 104. Orthogonaltransformation coefficient information 105, for example, DCT coefficientinformation is achieved in the orthogonal transformer 104. Theorthogonal transformation efficiency information 105 is quantized by aquantizer 106 to achieve quantized orthogonal transformation coefficientinformation 107. The quantized orthogonal transformation coefficientinformation 107 is input to both an entropy encoder 108 and an inversequantizer 109.

The quantized orthogonal transformation coefficient information 107input to the inverse quantizer 109 is successively subjected to theopposite processing to the processing of the quantizer 106 and theorthogonal transformer 104 by the inverse quantizer 109 and the inverseorthogonal transformer 110 to achieve a signal similar to the predictionerror signal, and then the signal is added with the predictive imagesignal 102 in an adder 110, thereby generating a local decoded imagesignal 112. The local decoding image signal 112 is input to the framememory/predictive image generator 108.

The frame memory/predictive image generator 113 generates the predictiveimage signal from the input image signal 100 and the local decoded imagesignal 112. The details of the frame memory/predictive image generator113 will be described later.

In the entropy encoder 108, the quantized orthogonal transformationcoefficient information 107 and motion vector information/predictionmode information 114 are subjected to entropy encoding. Respective codesthus generated are multiplexed by a multiplexer 116, and then smoothenedby an output buffer 117.

The encoded data 118 output from the output buffer 117 are transmittedto a transmission system or accumulation system (not shown).

A mode selector 121 directly controls a switch in the framememory/predictive image generator 113 to select a prediction mode, andalso controls the quantization parameter. The details of the modeselector 121 will be described later.

A coding controller 119 controls a coding portion 115 containingelements from the subtracter 101 to the frame memory/predictive imagegenerator 113. In this embodiment, a number of coded bits is allocatedevery coding unit while considering the buffer amount of the buffer 117.This embodiment uses a macroblock unit as the coding unit, for example,however, the invention is not limited to this mode.

(2) Frame Memory/Predictive Image Generator 113

FIG. 2 shows a specific example of the frame memory/predictive imagegenerator 113 for generating the predictive image signal 102 from theinput image signal 100 and the local decoded image signal 112.

The local decoded image signal 112 from the adder 111 of FIG. 1 istemporarily accumulated in a frame memory 200.

A motion vector detector 201 carries out the matching (block matching)between the input image signal 100 and the local decoded image signalaccumulated in the frame memory 200 every block in the frame to detect amotion vector.

An inter-frame predictor 202 subjects motion compensation to the localdecoded image signal in the frame memory 200 on the basis of the motionvector detected in the motion vector detector 201 to generate apredictive image signal based on the inter-frame prediction.

On the other hand, a intra-frame predictor 203 generates a predictiveimage signal based on the intra-frame prediction from the input imagesignal 100 and the local decoded image signal of an area which has beenalready coded in a frame of the frame memory 200.

The inter-frame predictor 202 has inter-frame prediction modes of K (Krepresents an integer of two or more), and the intra-frame predictor 203has intra-frame prediction modes of L (L represents an integer of two ormore). A switch 204 is connected to the outputs of the inter-framepredictor 202 and the intra-frame predictor 203. The switch 204 iscontrolled by the mode selector 121, and outputs a predictive imagesignal based on a prediction mode selected from the inter-frameprediction modes of K or a predictive image signal based on a predictionmode selected from the intra-frame prediction modes of L.

The motion vector information output from the motion vectorinformation/prediction mode 114, that is, the motion vector detector 201and the prediction mode information indicating the prediction modeselected in the switch 204 by the mode selector 121 are transmitted tothe entropy encoder 108. The motion vector information is output fromthe motion vector detector 201 only when the inter-frame prediction modeis selected.

(3) Coding Controller 119 and Mode Selector 121

FIG. 3 is a flowchart showing the flow of the operation of the codingcontroller 119 and the mode selector 121 in FIG. 1.

First, the coding controller 119 controls the coding portion 115 tosuccessively carry out the coding operation in plural prediction modes(the inter-frame prediction modes of K and the intra-frame predictionmodes of L) and also switch the quantization parameter in eachprediction mode.

An undetermined multiplier λ is calculated from the referencequantization parameter Q₀ given from the coding controller 109 (stepS301).

Subsequently, three reference quantization points are set to carry outthe coding operation in each prediction mode, and a set of number ofcoded bits R—coding distortion D pairs (hereinafter referred to as “R-Dpoint”) is created (step S302).

A lower left convex hull set is formed by the R-D point set thuscreated, and a prediction mode which does not exist on the lower leftconvex hull plane is deleted (step S303). Here, the “lower left convexhull set” will be described. A convex hull is formed from an R-D pointset existing on a two-dimensional orthogonal coordinate system in whichthe number of coded bits R is set to the abscissa axis and the codingdistortion D is set to the ordinate axis. The lower left convex hull setis an R-D point set that exists on a convex hull connecting the positionof the R-D point providing the minimum number of coded bits R in theconvex hull and the position of the R-D point providing the minimumcoding distortion D and also exists at a nearer side to the origin ofthe two-dimensional orthogonal coordinate system. The details will bedescribed later.

An R-D point of a quantization parameter which is not coded in theresidual R-D points is estimated(step S304).

An R-D point providing the minimum cost is selected from the R-D pointset and set as a new reference quantization parameter (step S305).

When coding is carried out at a predetermined coding frequency or it isimpossible to set the reference quantization point because coding hasbeen already carried out, the processing is finished, and the predictionmode and the quantization parameter providing the minimum cost that areselected in the step S305 are output. If not so, the processing returnsto step S302 to repeat the processing.

Next, specific examples of the processing of each of the steps S301 toS306 of FIG. 3 will be described.

[Step S301]

The undetermined multiplier λ is estimated from the referencequantization parameter Q₀ given from the coding controller 109 by usingthe following equation (1), for example.λ=f×exp(gQ ₀)  (1)

Here, f and g represent constants determined on the basis of theprediction structure of picture or slice, such as a picture type and aslice type.

[Step S302]

The coding is carried out on the basis of totally three quantizationparameters, that is, the reference quantization parameter Q₀ given fromthe coding controller 109 and the two quantization parameters Q_(min)and Q_(max) before and after the value of the reference quantizationparameter every prediction mode to determine a number of coded bits Rand a coding distortion D (see FIG. 4).

Here, the coding distortion D is calculated on the basis of the squareerror between an input image signal (org) and a local decoded imagesignal (coded) of each of the brightness signal Y and the colordifference signals Cb and Cr every macroblock. $\begin{matrix}{D = {{\sum\limits_{i = 0}^{15}{\sum\limits_{j = 0}^{15}( {{Y_{org}( {i,j} )} - {Y_{coded}( {i,j} )}} )}} + {\sum\limits_{i = 0}^{7}{\sum\limits_{j = 0}^{7}( {{{Cb}_{org}( {i,j} )} - {{Cb}_{coded}( {i,j} )}} )}} + {\sum\limits_{i = 0}^{7}{\sum\limits_{j = 0}^{7}( {{{Cr}_{org}( {i,j} )} - {{Cr}_{coded}( {i,j} )}} )}}}} & (2)\end{matrix}$In place of the square errors of the brightness signal Y and the colordifference signals Cb and Cr, the coding distortion may be the squareerror of only the bright signal, or any value such as an absolute valuedifferential sum, a value weighted on the basis of a visualcharacteristic or the like insofar as it is a value representing thecoding distortion.[Step S303]

Subsequently, a lower left convex hull is formed by the R-D point set,and a prediction mode which does not exist on the convex hull plane isdeleted.

The set of the lower left convex hull will be hereunder described on theR-D plane corresponding to the two-dimensional orthogonal coordinatesystem shown in FIG. 10 in which the number of coded bits R is set tothe abscissa axis and the coding distortion D is set to the ordinateaxis.

A convex hull comprising the set of the primary number of codedbits—coding distortion pairs is formed on the two-dimensional orthogonalcoordinate system. This convex hull is a set having a shape nearer to aclosed elliptical shape.

There is considered a set located on the convex hull connecting theposition A of the number of coded bits—coding distortion pair providingthe minimum number of coded bits R and the position B of the number ofcoded bits—coding distortion pair providing the minimum codingdistortion D. Since the shape of the convex hull is nearer to theelliptical shape, two kinds of sets, that is, a set existing at a sidenearer to the origin of the two-dimensional orthogonal coordinate system(the set of the lower left convex hull and it is indicated by a solidline of FIG. 10), and a set existing at a side farther from the origin(the set of the upper right convex hull and it is indicated by a dashedline) exist as the set on the convex hull.

Therefore, if the set of the number of coded bits—coding distortionpairs existing at the side near to the origin is a set of secondarynumber of coded bits—coding distortion pair, the set of the lower leftconvex hull could be selected.

The set existing at the side nearer to the origin will be furtherdescribed while considering the cost function (J=D+λ·R) described later.

First, the position A of the minimum number of coded bits paircorresponding to the number of coded bits—coding distortion pairproviding the minimum number of coded bits R in the convex hull isdetermined.

Subsequently, the position B of the minimum coding distortion paircorresponding to the number of coded bits—coding distortion pairproviding the minimum coding distortion D in the convex hull isdetermined.

Next, a straight line L (J=D+λ·R) passing through the position A of theminimum number of coded bits pair and the position B of the minimumcoding distortion pair thus determined is considered. When the gradientλ of the line L is represented by λ1 and the intercept J of the line Lis represented by J1, the condition: J1≧J=D+λ1·R is satisfied at thelower side of the lower left convex hull. Accordingly, the set of thenumber of coded bits—coding distortion pairs at which the value Q of thecost function determined by the common undetermined multiplier λ1corresponding to the above gradient is equal to or lower than the valueof the minimum number of coded bits pair or the minimum codingdistortion pair is set as the set of the secondary number of codedbits—coding pairs. This set corresponds to the set of the lower leftconvex hull.

A method of finding out the lower left convex hull as shown in FIG. 7will be described with reference to FIGS. 5 and 6.

First, the primary R-D point set is rearranged in an increasing ordecreasing order with the value of the coding distortion D as areference, for example. Actually, the coding distortion D of eachprediction mode is increased as the quantization parameter is larger.Therefore, the rearrangement is unnecessary, and the primary R-D pointset may be merged in increasing or decreasing order. FIG. 6 shows a casewhere the primary R-D points of FIG. 5 are ordered on the basis of thevalue of the coding distortion D, and the coding distortion D isincreased in the connection order of the primary R-D points connected toone another by a broken line.

Next, as shown in FIGS. 6A and 6B, each point of the primary R-D pointset is successively added with the next one point in the increasingorder or decreasing order of the coding distortion D, and the shape of abroken line connecting the latest three points is checked. Here, if theshape of the broken line is a convex shape under the view from the lowerleft side as shown in FIG. 6A, the three points are left as points ofthe secondary R-D point set, and the next point is added. On the otherhand, if the shape of the broken line is a concave shape under the viewfrom the lower side as shown in FIG. 6B, the point in the middle of thethree points is deleted.

The processing as described above is carried out on all the points ofthe R-D point set, and the residual R-D points at the time when theprocessing concerned is finished are set as a lower left convex hullplane set. By checking the shape of the broken line at the frequencycorresponding to the number of all the points, it can be judged whetherthe processing on all the points of the R-D point set is finished.

At this time, with respect to prediction modes which never belong to thelower left convex hull plane set, they are deleted from the subsequentcandidates of the prediction modes. Specifically, a mode 1 and a mode 4shown in FIG. 7 never belong to the lower left convex hull plane set,and thus they are deleted from the candidates at this time point.

[Step S304]

Subsequently, with respect to the residual prediction modes, the numberof coded bits R and coding distortion D of a quantization parameterwhich has not been coded are estimated from the points of the actuallycoded quantization parameter before and after the non-coded quantizationparameter concerned as shown in FIG. 8. It is assumed that the number ofcoded bits R and the coding distortion D satisfy the relationship shownby the equations (3) and (4) with the quantization parameter representedby Q. In the following equations, a, b, c, d represents coefficients.logR=aQ+b  (3)logD=cQ+d  (4)

The coefficients a, b, c, d assumed in the equations (3) and (4) areestimated from the following equations (5) to (8) by using the numbersof coded bits R1, R2 and the coding distortions D1, D2 of theactually-coded two points before and after the non-coded quantizationparameter. $\begin{matrix}{a = \frac{{\log\quad R_{2}} - {\log\quad R_{1}}}{Q_{2} - Q_{1}}} & (5) \\{b = \frac{{Q_{2}\log\quad R_{1}} - {Q_{1}\log\quad R_{2}}}{Q_{2} - Q_{1}}} & (6) \\{c = \frac{{\log\quad D_{2}} - {\log\quad D_{1}}}{Q_{2} - Q_{1}}} & (7) \\{d = \frac{{Q_{2}\log\quad D_{1}} - {Q_{1}\log\quad D_{2}}}{Q_{2} - Q_{1}}} & (8)\end{matrix}$From these coefficients a, b, c, d, the number of coded bits R and thecoding distortion D of each quantization parameter which skip somevalues are estimated from the four coefficients a, b, c, d according tothe following equations (9) and (10).{circumflex over (R)}(Q)=exp(aQ+b)  (9){circumflex over (D)}(Q)=exp(cQ+d)  (10)Referring to FIG. 8, white circular points represent actually measuredvalues, and white rectangular points represent estimation valuesestimated in the above step. The points thus estimated are also added tothe R-D point set.[Step S305]

Subsequently, the R-D point providing the minimum cost is selected fromthe R-D point set. Here, the cost calculation of each point is carriedout according to the cost function of the equation (11). λ representsthe undetermined multiplier determined in the step S301.J=D+λR  (11)

The quantization parameter of the point providing the minimum cost inthe R-D point set as shown in FIG. 9 is set as a new referencequantization parameter Q₀.

[Step S306]

In this case, when the coding frequency for the R-D point calculationexceeds a predetermined frequency, or when the new referencequantization parameter corresponding to a point which has been alreadycoded, it is judged that the condition is satisfied, the processing isfinished, and the quantization parameter and the prediction mode of thepoint selected in step S305 are output.

If the condition is not satisfied, the processing returns to step S302,and the processing of steps S302 to S306 is repeated until the abovecondition is satisfied.

(4) Modification

In the above embodiment, the macroblock is set as the coding unit, and apair of prediction mode and quantization parameter is determined atevery macroblocks. However, the coding unit may be set to pluralmacroblocks, or it may be set to another unit such as slice, field,frame, picture or GOP.

Furthermore, the above embodiment has been described by taking themotion picture coding as an example. However, the invention may beapplied to still picture coding.

1. An image coding control method for controlling a quantizationparameter by using a prediction mode selected from plural predictionmodes every any coding unit, comprising: calculating an undeterminedmultiplier serving as a parameter used for a cost function from a givenreference quantization parameter; selecting a set of secondary number ofcoded bits—coding distortion pairs from a set of primary number of codedbits—coding distortion pairs that correspond to plural combinations ofthe prediction mode and the quantization parameter every coding unit;selecting number of coded bits—coding distortion pairs providing theminimum cost function as the optimal number of coded bits—codingdistortion pairs from the set of the secondary number of codedbits—coding distortion pairs by using the undetermined multiplier thuscalculated every coding unit; and determining a prediction mode and aquantization parameter used for the coding from the optimal number ofcoded bits—coding distortion pairs, wherein when the set of thesecondary number of coded bits—coding distortion pairs is selected everycoding unit, (1) a convex hull comprising the set of the primary numberof coded bits—coding distortion pairs is formed on a two-dimensionalorthogonal coordinate system in which the number of coded bits is set asone axis and the coding distortion is set as the other axis, (2) a seton the convex hull that connect the position of a number of bits—codingdistortion pair providing the minimum number of coded bits and theposition of a number of coded bits—coding distortion pair providing theminimum coding distortion is determined, and (3) a set of number ofcoded bits—coding distortion pairs that is on the convex hull and existsat a nearer side to the origin of the two-dimensional orthogonalcoordinate system is selected as a set of the secondary number of codedbits—coding distortion pairs.
 2. The image coding control methodaccording to claim 1, wherein when the set of the secondary number ofcoded bits—coding distortion pairs is selected every coding unit, (1) aminimum number of coded bits pair corresponding to the number of codedbits—coding distortion pair providing the minimum number of coded bitsis determined, (2) a minimum coding distortion pair corresponding to thenumber of coded bits—coding distortion pair providing the minimum codingdistortion is determined; (3) a convex hull comprising the set of theprimary number of coded bits—coding distortion pairs on atwo-dimensional orthogonal coordinate system in which the number ofcoded bits is set to one axis and the coding distortion is set to theother axis is determined, (4) the value of a cost function determined bya common undetermined multiplier between the minimum number of codedbits pair and the minimum coding distortion pair and existing on theconvex hull is calculated, (5) it is judged whether the value of thecost function is not more than the value in the minimum number of codedbits pair or the minimum coding distortion pair, and (6) a set of thenumber of coded bits—coding distortion pairs in which the value of thecost function is not more than the value concerned is selected as a setof secondary number of coded bits—coding distortion pairs.
 3. The imagecoding control method according to claim 1, wherein when the set of thesecondary number of coded bits—coding distortion pairs is selected everycoding unit, the set of the primary number of coded bits—codingdistortion pairs located in the convex hull is deleted from thecandidate of the set of the secondary number of coded bit—codingdistortion pairs.
 4. The image coding control method according to claim1, wherein the coding unit is set to a macroblock unit, a unit of pluralmacroblocks, a slice unit, a field unit, a frame unit, a picture unit orGOP unit.
 5. An image coding control device for controlling aquantization parameter by using a prediction mode selected from pluralprediction modes every any coding unit, comprising: an undeterminedmultiplier calculating processor unit for calculating an undeterminedmultiplier serving as a parameter used for a cost function from a givenreference quantization parameter; a set selecting processor unit forselecting a set of secondary number of coded bits—coding distortionpairs from a set of primary number of coded bits—coding distortion pairsthat correspond to plural combinations of the prediction mode and thequantization parameter every coding unit; an optimal set selectingprocessor unit for selecting number of coded bits—coding distortionpairs providing the minimum cost function as the optimal number of codedbits—coding distortion pairs from the set of the secondary number ofcoded bits—coding distortion pairs by using the undetermined multiplierthus calculated every coding unit; and a determining processor unit fordetermining a prediction mode and a quantization parameter used for thecoding from the optimal number of coded bits—coding distortion pairs,wherein when the set of the secondary number of coded bits—codingdistortion pairs is selected every coding unit, the set selectingprocessor unit (1) forms a convex hull comprising the set of the primarynumber of coded bits—coding distortion pairs on a two-dimensionalorthogonal coordinate system in which the number of coded bits is set asone axis and the coding distortion is set as the other axis, (2)determines a set on the convex hull that connect the position of anumber of coded bits—coding distortion pair providing the minimum numberof coded bits and the position of a number of coded bits—codingdistortion pair providing the minimum coding distortion, and (3) selectsa set of number of coded bits—coding distortion pairs that is on theconvex hull and exists at a nearer side to the origin of atwo-dimensional orthogonal coordinate system, as a set of the secondarynumber of coded bits—coding distortion pairs.
 6. The image codingcontrol device according to claim 5, wherein when the set of thesecondary number of coded bits—coding distortion pairs is selected everycoding unit, the set selecting processor unit (1) determines a minimumnumber of coded bits pair corresponding to the number of codedbits—coding distortion pair providing the minimum number of coded bits,(2) determines a minimum coding distortion pair corresponding to thenumber of coded bits—coding distortion pair providing the minimum codingdistortion; (3) determines a convex hull comprising the set of theprimary number of coded bits—coding distortion pairs on atwo-dimensional orthogonal coordinate system in which the number ofcoded bits is set to one axis and the coding distortion is set to theother axis, (4) calculates the value of a cost function determined by acommon undetermined multiplier between the minimum number of coded bitspair and the minimum coding distortion pair and existing on the convexhull, (5) judges whether the value of the cost function is not more thanthe value in the minimum number of coded bits pair or the minimum codingdistortion pair, and (6) selects a set of the number of codedbits—coding distortion pairs in which the value of the cost function isnot more than the value concerned, as a set of secondary number of codedbits—coding distortion pairs.
 7. The image coding control deviceaccording to claim 5, wherein the set of the primary number of codedbits—coding distortion pairs located in the convex hull is deleted fromthe candidate of the set of the secondary number of coded bits—codingdistortion pairs.
 8. The image coding control device according to claim5, wherein the coding unit is set to a macroblock unit, a unit of pluralmacroblocks, a slice unit, a field unit, a frame unit, a picture unit orGOP unit.
 9. An image coding control program product for controlling aquantization parameter through a computer by using a prediction modeselected from plural prediction modes every any coding unit, the programproduct comprising instructions of: calculating an undeterminedmultiplier serving as a parameter used for a cost function from a givenreference quantization parameter; selecting a set of secondary number ofcoded bits—coding distortion pairs from a set of primary number of codedbits—coding distortion pairs that correspond to plural combinations ofthe prediction mode and the quantization parameter every coding unit;selecting number of coded bits—coding distortion pairs providing theminimum cost function as the optimal number of coded bits—codingdistortion pairs from the set of the secondary number of codedbits—coding distortion pairs by using the undetermined multiplier thuscalculated every coding unit; and determining a prediction mode and aquantization parameter used for the coding from the optimal number ofcoded bits—coding distortion pairs, wherein when the set of thesecondary number of coded bits—coding distortion pairs is selected everycoding unit, the instruction of selecting the set of secondary number ofcoded bits—coding distortion pairs (1) forms a convex hull comprisingthe set of the primary number of coded bits—coding distortion pairs on atwo-dimensional orthogonal coordinate system in which the number ofcoded bits is set as one axis and the coding distortion is set as theother axis, (2) determines a set on the convex hull that connect theposition of a number of coded bits—coding distortion pair providing theminimum number of coded bits and the position of a number of codedbits—coding distortion pair providing the minimum coding distortion, and(3) selects a set of number of coded bits—coding distortion pairs thatis on the convex hull and exists at a nearer side to the origin of thetwo-dimensional orthogonal coordinate system, as a set of the secondarynumber of coded bits—coding distortion pairs.
 10. The imagecoding-control program product according to claim 9, wherein when theset of the secondary number of coded bits—coding distortion pairs isselected every coding unit, the instruction of selecting the set ofsecondary number of coded bits—coding distortion pairs (1) determines aminimum number of coded bits pair corresponding to the number of codedbits—coding distortion pair providing the minimum number of coded bits,(2) determines a minimum coding distortion pair corresponding to thenumber of coded bits—coding distortion pair providing the minimum codingdistortion; (3) determines a convex hull comprising the set of theprimary number of coded bits—coding distortion pairs on atwo-dimensional orthogonal coordinate system in which the number ofcoded bits is set to one axis and the coding distortion is set to theother axis, (4) calculates the value of a cost function determined by acommon undetermined multiplier between the minimum number of coded bitspair and the minimum coding distortion pair and existing on the convexhull, (5) judges whether the value of the cost function is not more thanthe value in the minimum number of coded bits pair or the minimum codingdistortion pair, and (6) selects a set of the number of codedbits—coding distortion pairs in which the value of the cost function isnot more than the value concerned, as a set of secondary number of codedbits—coding distortion pairs.
 11. The image coding control programproduct according to claim 9, wherein the instruction of selecting theset of secondary number of coded bits—coding distortion pairs comprisesan instruction of deleting the set of the primary number of codedbits—coding distortion pairs located in the convex hull from thecandidate of the set of the secondary number of coded bits—codingdistortion pairs.
 12. The image coding control program product accordingto claim 9, wherein the coding unit is set to a macroblock unit, a unitof plural macroblocks, a slice unit, a field unit, a frame unit, apicture unit or GOP unit.